[chainstate] Soft fork kit -- signaling infrastructure

[jcnelson]

As part of implementing SIP 011, the Stacks nodes need a way to identify and signal for soft forks, as well as a way to identify Stacks blocks that they either must or must not build off of. This issue is for soft fork signaling.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed. Please reopen if needed.

[blockstack-devops]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.